[luatex] How to get a \mid binary relation that grows in LuaTeX

[Hans Hagen]

On 11/17/2017 11:35 AM, Ulrike Fischer wrote:
> Am Thu, 16 Nov 2017 10:28:56 +0100 schrieb Hans Hagen:
> 
>>> Can one query the number by name? So that
>>> \Umiddle class \directlua{whatever(rel)}  gives now 5 and in the
>>> next version 3?
> 
>> these numbers are documented part of traditional tex, so we use that
>> 'rel' number now (the internal ones are just that: internal)
> 
> My question was how to write code that works in a current texlive
> (where you need \Umiddle class 5 to get relation spacing) *and* in a
> future luatex where class 3 will be the correct value.
> 
> Is querying the luatex version the only way?
Yes (unless you want to use the status.development_id variable which we 
use when testing new functionality but there is no formal description of 
what was changed cq. is supported at a specific id other than the svn 
comments but in this case you could use something

\ifnum \directlua{tex.print(status.development_id)} > 6471

or so. We don't bump version numbers too often so updating bwtween tex 
live releases is up to the users i.e. when they update the engine they'd 
better also update their macro package that depends on it

Hans

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